Electric fluid dispensers have been developed for dispensing applications requiring a precise placement of a fluid, for example, an adhesive, onto a moving substrate, for example, packaging or a woven product. Dispensing guns of this type include a liquid passage that communicates between a pressurized adhesive supply and a valve mechanism provided at the end of the liquid passage. The valve mechanism is typically a movable valve stem positioned to selectively obstruct a dispensing orifice formed in a valve seat. The valve stem is extended and retracted relative to the valve seat in a controlled manner by a solenoid for providing repeatable and accurate dispense patterns of the liquid onto the moving substrate. Generally, the solenoid comprises an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole, thereby moving the valve stem. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve stem toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
Dispensing systems have been developed that employ driver circuits to control the operation of the solenoid within the dispensing gun. To open the valve, the driver circuit applies a fast pull-in current to the solenoid coil to quickly retract the valve stem and open the dispensing orifice at the beginning of a dispensing cycle. The driver circuit maintains a minimal holding current which holds the valve stem in an open position while minimizing the amount of heat build-up in the solenoid coil during the dispensing cycle. Finally, the driver circuit provides a fast demagnetization of the solenoid so the valve stem is quickly closed over the orifice at the end of the dispensing cycle.
Closing of the valve stem is often achieved by a spring mechanism connected to one end of the valve stem. When the solenoid is sufficiently demagnetized, the stored energy in the compressed spring mechanism forces the valve stem to the closed position and in sealing engagement with the dispensing orifice. One example of such a dispensing system is set forth in U.S. Pat. No. 5,812,355, owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference in its entirety.
In unregulated gun drivers, current to the electric gun coil is supplied by a power switching circuit that is connected to an unregulated power supply. Thus, any variations in line voltage changes the output voltage from the power supply which is applied to the power switching circuit. Changing the voltage applied to the power switching circuit results in a corresponding variation in the current being supplied to the gun solenoid. The operational speed of the solenoid is directly related to the magnitude of the applied voltage; and therefore, as the magnitude of the applied voltage goes up, the armature and valve stem move faster. Similarly, as the magnitude of the applied voltage goes down, the armature and valve stem move slower. Thus, the operational speed of the armature and valve stem is related to the magnitude of the voltage applied to the coil and hence, the actuation time or time required to open and close the electric gun is changed by variations in line voltage applied to the unregulated power supply.
Uncontrolled and unpredictable variations in the actuation time of the dispensing gun adversely impact the adhesive deposition process. Line voltage variations changing the actuation time of the dispensing gun also change the starting and stopping locations of the dispensed adhesive on the substrate. If adhesive is to be dispensed on a package flap moving past the dispensing gun, an increase in line voltage causing the gun to switch-on or open faster than expected may cause adhesive to be dispensed too soon. Opening the gun too soon may cause adhesive to be dispensed prior to a leading edge of the flap reaching the dispensing location. Similarly, a decrease in line voltage causes the gun to switch-off or close slower than expected. This slower gun operation may cause adhesive to continue to be dispensed after a trailing edge of the flap passes the dispensing location. Any unpredicted dispensing of adhesive onto a surface not intended to receive adhesive, potentially results in a scrap product. In addition, spurious adhesive spray that misses the substrate may lead to additional, time consuming, labor intensive and expensive cleaning and maintenance of equipment and areas adjacent the adhesive dispensing gun. Thus, such line voltage variations may result in a less efficient, less economical and/or lower quality fluid dispensing operation.
It is known to use a regulated gun driver, that is, a gun driver with a regulated power supply. A regulated gun driver provides a constant voltage to the coil independent of the voltage variations to the power switching circuit. Thus, with respect to line voltage variations, the use of a regulated gun driver provides a more consistent dispensing gun performance. However, regulated gun drivers are more expensive than unregulated gun drivers and create more heat which requires more cooling and thus, further adds to their cost.
Therefore, there is a need to provide an electric fluid dispenser that uses an unregulated solenoid gun driver that is insensitive to variations in the applied line voltage.